Chapter 3 Adler: Individual Psychology 97who had not participated in any civil disobedience or among a control group of
college students simply drawn from classes at the same college. These findings lend
empirical support for the prediction of greater conventionality and cooperativeness
among eldest children, compared to their more radical, risk-taking younger siblings.
Although Sulloway has been criticized for his methodology (he collected bio-
graphical data on historical individuals), Born to Rebel breathed new life into birth
order research, and since its publication, many and better studies have been con-
ducted to test Adler’s predictions. Generally, “between-family” research designs
(individuals from different families are compared) tend not to confirm Adler’s
theory, perhaps because of the difficulty in these sorts of designs of controlling for
the many variables that distinguish families. In contrast, “within-family” designs
ask respondents to compare themselves to their own siblings, and these studies are
more likely to provide some confirmation for Adler’s theory. For example, Paulhus,
Trapnell, and Chen (1999) conducted a within-family study of over 1,000 families
and found that first-borns were nominated as most high-achieving and conscien-
tious, while later-borns were seen as most rebellious, liberal, and agreeable. In a
very recent review of over 200 birth-order studies that did show significant differ-
ences between siblings, Eckstein and colleagues (2010) found support for Adler
and Sulloway: first-borns and only children are seen as the most high-achieving,
and later-borns as the most rebellious and socially interested.
Important to note in Adler’s theory of birth order effects is that he hypothesized
that it is the family constellation, not something biological or prenatal, that results
in different personalities among siblings. That is, for Adler, the personalities of later-
borns are shaped by older siblings’ and parents’ attitudes toward and treatment of
them. But is this social hypothesis supported by the research? Research using within-
family designs has been conducted to examine both hypotheses. One important bio-
logical theory concerns immunoreactivity. The theory is that histocompatibility-Y
antigens, which are exclusive to males and located on the Y-chromosome, can induce
an immune system response from mothers carrying male fetuses. Several studies have
examined the immunoreactivity hypothesis for explaining sexual orientation in males,
with evidence that the prevalence of homosexuality among males is higher among
later-born males with a greater number of older brothers (e.g., Bogaert & Skorska,
2011). The reason the finding is explained biologically is that studies show that
fraternal birth order does not predict homosexuality in males in adoptive or blended
families, only in biologically related brothers (Bogaert, 2006).
On the other hand, greater support has been found for other birth order effects,
such as higher achievement among first- than later-borns, being explained by the more
Adlerian social, family constellation perspective. For example, one very recent study
used data on fully adopted sibling and fully biologically related sibling groups in a
within-family design of Swedish sibling sets. Barclay (2015) found that older chil-
dren indeed had higher educational attainment than their later-born siblings, regard-
less of whether they were from adoptive or biologically related families. This finding
provides strong evidence for intrafamily dynamics such as resource competition,
much as Sulloway predicted, being the driving force behind birth order effects.
In general, the specific predictions that Adler made about the traits of oldest,
middle, youngest and only children have not found strong support in the research lit-
erature. However, within-family studies have indeed found support for some consistent